The major objective of this proposal is to identify the nephron sites of altered sodium reabsorption in experimental models of chronic sodium retention. The effect of acute expansion of the extracellular and vascular volumes on sodium reabsorption by the proximal tubule, loop of Henle and distal tubule will be studied in control dogs and dogs with chronic sodium retention. Experimental models will include the chronic administration of mineralocorticoids and the production of experimental heart failure. Micropuncture techniques will be used to study these problems.